In recent years, an ultraviolet light irradiation method has been widely used to sterilize microorganisms such as filamentous fungi, bacteria, and viruses contained in atmosphere or a liquid such as water. Irradiation with especially C wave (UVC) at a wavelength of 280 nm or less among ultraviolet light is said to cause disruption of a function of DNA replication of viruses and exhibit a really high effect of killing viruses.
For the above-described reasons, a low-pressure mercury lamp which efficiently emits ultraviolet light at 254 nm has been widely used as a light source for sterilization and has been commercialized.
To efficiently use output of the light source, a method in which a low-pressure mercury lamp is placed in a cell for sterilization as a light source and the inside of the cell is coated with a highly reflective material has been proposed (e.g., see PTL 1).
However, the low-pressure mercury lamp has a problem in that the exchange frequency is high because of the short service life as a light source, and it takes time and effort to maintain. Moreover, when the low-pressure mercury lamp is used as a light source, the size of the whole cell including a power unit and the like is increased, and it is difficult to downsize the cell.
Therefore, a method using, as a light source, an ultraviolet LED as a substitute for a low-pressure mercury lamp has been proposed. An LED has a long service life and is small and light, and thus it is easy to downsize the cell (e.g., PTL 2). However, optimization of a material of a cell into which an object to be irradiated (fluid) is introduced to enhance sterilization efficiency in the method using an ultraviolet LED as a light source has not been sufficiently studied yet.